You need to know how the servo valve on the PFCU works. This is very difficult to explain without a diagram of the PFCU. Hopefully your textbooks have one. Very basically, the pilot moves the control column, on "conventional" controlled aircraft this moves the appropriate flight control cable which in turn moves the piston in the servo valve on the PFCU. Hydraulic pressure is then ported such that the main ram moves in the appropriate direction relative to the casing. The casing itself is the output lever to which the flight control surface is connected, and therefore the control surface will deflect in the required direction. Fluid on the other side of the balanced ram is free to return, via ports in the servo, to the reservoir return line. Due to the fact that the main ram is attached at one end to the airframe, the hydraulic pressure has to move the entire PFCU body, and if there is no change in position of the servo valve piston (i.e. no fresh input from the pilot) the movement of the PFCU body will itself close off the outlet ports of the servo valve, and hydraulic fluid will cease to flow at the correct control surface deflection. In this situation the main ram is hydraulically locked, with fluid of equal pressure on both sides, and neither the control surface or PFCU body will move, until of course the equilibrium is altered by a new control input moving the servo piston again.
Hope that helps!!!
Last edited by basil fawlty; 17th April 2002 at 00:38.